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Why Virtual Short Concept in an Op-Amp is only applicable to negative feedback

HARI T O1
Expert 1645 points

Dear Team,

I am a great fan of TI Precession Labs opamp's tutorial.

I have a question which is haunting me for a long time.Till now I did not get any proper explanation.

If you can explain with mathematical proof it is appreciated.

The virtual short concept is only applied to op-amps in negative feedback configuration, and not in case of positive feedback.

Is there any plausible reason for this?

My expectation is that mathematical proof which explains why it is applicable to negative feedback and not applicable to positive feedback

Regards

HARI

  • 0
    TI__Guru**** 164436 points

    Hello Hari,

    Draw and post a positive feedback circuit and I'll tell you why there is or is not a virtual short.

  • 0 in reply to Ron Michallick
    Expert 1645 points

    Hi Ron,

    Thank you very much.

    Please see the positive feedback circuit below.

    Negative feedback circuit is shown below.

    Regards

    HARI

  • 0 in reply to HARI T O1
    TI__Guru**** 164436 points

    Hari,

    Thought experiment first for top schematic. Let's assume that VIN, IIN, VOUT, and IF are all zero at time zero. This is a potential balanced point with a potential virtual short. Now VIN rises which will apply positive voltage on IN+ because VOUT can not change instantly. Input voltage difference is positive. Op amp gain is positive. So output will start to go positive; RF will increase positive voltage on IN+ further. Eventually VOUT reaches maximum voltage VOH (after slew rate time). Both VIN and VOUT are positive so IN+ is not zero.  

    Research hysteresis comparator; that is the top schematic.

  • 0 in reply to Ron Michallick
    Expert 1645 points

    Hi Ron,

    Thank you very much.

    I know this is a comparator and it's in positive feedback.

    The inverting input is grounded and I am not able to explain a third person why non inverting input is not 0V.

    Regards

    HARI

  • 0 in reply to HARI T O1
    TI__Guru**** 164436 points

    Math and image taken from sloa011b, good to read all of it 

    What happens when op amp gain '-a' becomes a big negative number. circuit gain 'A' becomes a very big negative number that is not set by the application two resistors. In summary you can't just swap the inputs and expect a similar results in a close loops system.